Examples of such a conventional apparatus of this type include an apparatus that performs printing on printing paper by ejecting ink droplets from each of nozzles in an inkjet head while moving the printing paper relative to the inkjet head. An inkjet printing apparatus typically includes an inkjet head with a plurality of minute nozzles from each of which ink droplets are ejected. Consequently, in the inkjet printing apparatus, nozzle clogging may occur due to dust or increased viscosity of the ink droplets. Performing printing in the state above may cause no ejection of ink droplets. This leads to some white lines on the printing paper. The printing paper with such white lines is regarded as defective and to be discarded.
Then, the conventional inkjet printing apparatus performs an ejection test for detecting a non-ejection nozzle. When a non-ejection nozzle is detected, purge for discharging ink droplets from the nozzle by suction or pressure is performed to clean the nozzle. The purge eliminates the unejected ink droplets, and thus printing enables to be performed normally with a nozzle through which ink droplets are normally ejected.
The ejection test above is for example performed as under.
A light receiver and a light projector are disposed in face-to-face relationship in a direction where a plurality of nozzles is arranged. Then ink droplets are ejected from each of the nozzles in turn and are to be detected by the light receiver. From conditions detected by the light receiver, it is detected whether or not the ink droplets are ejected. When a non-ejection nozzle is found, the nozzle is cleaned. See, for example, Japanese Patent Publications No. H10-119307A, 2001-113725A, and 2003-127430A.
Moreover, when an inkjet head is provided that ejects ink heated by a heater disposed in the nozzle, infrared rays are emitted from the ink droplets, and thus an infrared sensor detects a non-ejection nozzle. See, for example, Japanese Patent Publication No. 2004-42281A.
Moreover, when a plurality of inkjet heads is provided, an ejection test is performed while a light projector, a light receiver, and a light reflector are disposed. See, for example, Japanese Patent Publication No. 2005-186381A. Furthermore, a light projector and a light receiver are disposed orthogonally relative to a direction where the nozzles of the inkjet head are arranged. An ejection test enables to be performed during printing. See, for example, Japanese Patent Publication No. 2006-240119A.
The examples of the conventional apparatus with such the constructions, however, have the following drawbacks. That is, in the conventional apparatus, the ejection test is performed and when a non-ejection nozzle is found, the non-ejection nozzle is cleaned independently of the number of non-ejection nozzles. As a result, a proportion of maintenance to a starting time may increase in the inkjet printing apparatus. This may lead a drawback that availability of the inkjet printing apparatus decreases.
This invention has been made regarding the state of the art noted above, and its object is to provide an inkjet printing apparatus and a nozzle cleaning method of the apparatus. The apparatus allows a decreased proportion of maintenance to a starting time in the apparatus and thus increased availability of the apparatus by performing cleaning in accordance with results of an ejection test with variations in size of ink droplets.